Ovens of the kind having a conveyor running beneath overhead transverse gas burners are widely used for continuous production of bread, pies, pizzas, biscuits and other baked foodstuffs.
The gas burners comprise tubes with side slots. Gas fed from one end burns, as a ribbon flame, along the side slots. In the case where a uniform flame is used along the length of the tube it is difficult to attain uniformity of heating since the edges of the conveyor tend to have different thermal transfer characteristics compared with the central region, typically giving greater heating at the edges.
Lack of uniformity is disadvantageous because this means that foodstuffs will not be equally baked across the conveyor. In an attempt to overcome this problem it is known to use a burner tube with two edge slots and one central slot all of equal length and communicating with separate internal compartments fed with gas via respective feed pipes and control valves. In this way it is possible to produce different flame sizes, and hence different heat intensities, at the edges compared with the central region. However, this does not provide a total solution to the problem because the change in thermal transfer characteristics across the conveyor will not normally occur in three distinct regions corresponding to the positioning of the three burner tube slots.
A further problem arises in connection with the attainment of high thermal output along the length of the three-compartment gas burner. The three compartments are fed with air and gas mixed with a Venturi injector. The ratio of the air to gas, and the flow rate of the mixture determine the thermal output per unit length of the gas burner slots. In the context of the short length slots of the three compartments, an inconvenient limitation may be imposed on the maximum flow rate and hence the maximum thermal output due to destabilisation of the flame. As the flow rate is increased by increasing mixture pressure and/or reducing slot width, there is an increasing tendency for the flame to `blow away` or otherwise destabilise. By way of example, whereas a 90,000 BTU/hr (95,000 KJoules/hr output may be attainable with a single zone 39 inch (99 cm) burner, it may be necessary to turn down the flow rate with a three-compartment burner of the same overall length so that a maximum output of only 45,000 BTU/hr (47,500 KJ/hr) is attainable.
Three-compartment burners may require complex and costly control systems for switching off the flames e.g. during gaps and product change-overs.